The invention relates to a method and device for manufacturing elastomer mixtures (elastomer compounds) to fabricate rubber articles, in which at least one elastomer to be processed for manufacturing the elastomer mixture is metered into a mixing extruder, and mixed and plasticized and/or masticated (“melted open”) while passing through the mixing extruder.
Elastomer mixtures can include a continuous elastomer matrix and a filler incorporated into the elastomer matrix, and are starting products for rubber manufacture. For example, the unprocessed elastomer can be natural rubber or synthetic rubber, e.g., nitryl rubber. Soot particles or silicate particles can be used as the filler, for example. The degree of distribution and degree of dispersal (degree of dispersion) have been shown to considerably influence the technical properties of the rubber. In known methods for manufacturing such elastomer mixtures, closed mixers are used for dispersive and distributive mixing along with plasticizing and/or mastication of the filler or elastomer.
To simplify the manufacture of elastomer mixtures for the rubber industry, an attempt was made to replace the traditional forms of presentation of natural or synthetic rubber, e.g., ball form, with liquid or powdery elastomer as the raw material. In addition, the particle form of presentation is a precondition for continuous rubber mixture manufacture.
The use of liquid elastomers has thus far only been rudimentarily developed, since this would require a rethinking and considerable investment on the part of the rubber industry. However, it would seem that powdery elastomers are proving themselves as a starting material for the manufacture of elastomer mixtures in both discontinuous and continuous procedures. In turn, this made it necessary to provide methods for manufacturing powdery rubber. Methods were developed for spray-drying and freeze-drying of nitryl rubber or natural rubber. The flash evaporation method or milling procedures were also used to generate smaller particles. However, one disadvantage to all of these methods lies in the stickiness and creeping characteristic of the generated rubber particles. Nonetheless, powder products were fabricated by dusting the particles with very fine organic and inorganic powders to deactivate the cohesive effects. Unfortunately, undesired contaminants were introduced into the manufactured particles time and again as a result of these measures.
As a consequence, a method was developed for manufacturing rubber/filler mixtures in powder form. In this case, a stable rubber/water emulsion is first formed independent of the type and state of the rubber (e.g., solution or emulsion polymerizates, natural rubber or nitryl rubber). This emulsion receives a filler (soot or silicate) whose particle size distribution had previously been precisely set, and which exists in an aqueous suspension along with various additives. The latex emulsion and filler suspension were homogenized in a mixing container via intensive blending. The precipitation process is then concluded under continuous blending in a reaction container, and the obtained precipitation product is transferred to a homogenization container for further processing. The bulk of the water is then removed in a centrifuge, whereupon the finely distributed mixture is dried to a residual water content of less than 1%. The resultant rubber/filler mixtures obtained in a powder form are free-flowing after drying, and suitable for the manufacture of rubber products in a continuous procedure.
To manufacture the rubber or rubber articles, the powdery rubber/filler mixtures obtained in this way are then plasticized and/or masticated (“melted open”) in a twin-shaft extruder along with softeners and other admixtures, which yields a continuous, liquid polymer matrix with filler particles suspended therein. Adding a vulcanizing agent creates additional crosslinkages between the elastomer molecules, so that the subsequently cooled “mass” has the desired rubbery properties. This makes it possible to fabricate tailor-made rubber mixtures depending on the type of used elastomer, and on the type, size and content of filler particles. Required to this end are at least two different powdery rubber/filler mixtures, from which a wide range of rubber mixtures can in turn be obtained by establishing a suitable mixture ratio between them.
EP-1035155A1 discloses such a method for manufacturing powdery, free-flowing rubber/filler mixtures (rubber/filler batches). The rubber and soot are precipitated to “granulates” (powdered rubber) via coagulation in a suspension with the corresponding rubber and soot. The powdered rubber has a round shape with a diameter of 1 to 3 mm, wherein the soot envelops the polymer. The soot is already present in a pre-distributed form. The soot enables easy handling, in particular simple storage and metering.
This method can basically be used to manufacture all important rubbers (natural or synthetic) as granulate. Various additives and softeners can be incorporated into the granulate, thereby simplifying the metering of components into the mixing aggregate.
However, it has also been shown that there are disadvantages to manufacturing rubber using powdery rubber/filler mixtures too. Specifically, an examination of the entire energy balance from natural rubber or artificial to the end product shows that this type of rubber manufacture is energy-intensive. Sine the water is already removed while manufacturing the powdered rubber (rubber/filler mixture), and this powdered rubber is subsequently “melted open” in a twin-shaft extruder along with a vulcanizing agent and other admixtures, a very high level of energy must be expended for melting open and incorporating the fillers. In addition to the high energy costs, care must here always be taken that the temperature of the product in the extruder does not exceed a critical temperature, since the elastomer might otherwise become damaged. The elastomer polymer chains are here broken down into smaller fragments and/or chemically modified by the high temperature, and/or additional organic constituents, in particular softeners, are broken down, and/or premature crosslinkage can be initiated in the presence of vulcanizing agent.